This invention relates to an imaging single lens, and in particular to an imaging single lens having a non-spherical surface which is suitably used where the light used is substantially monochromatic.
Optical information recording has become widely used as one of information recording methods. As an example of the optical information recording, mention may be made of a method of the type in which a number of two kinds of recording pits different in light reflectance are formed as binary-encoded digital signal rows on a recording medium such as a disc, a card or a tape. During the reproduction of such optically recorded information, the recording medium is illuminated by a suitable light source, the light from the recording pits of the recording medium is imaged on a photosensor by an imaging optical system, and the imaging optical system and the photosensor are moved relative to the recording medium, whereby a reproduction signal can be time-serially obtained from the photosensor.
Where a card is used as the information recording medium, a number of rows comprising a plurality of recording pits are arranged and formed and during reproduction, the light from a pit row is imaged on a photosensor, for example, a one-dimensional CCD, by an imaging optical system, whereby simultaneous reading of a pit row is effected. Therefore, the imaging optical system is required to have a good imaging performance over a relatively wide field of view.
Now, in the optical information recording as described above, the information recording density can be made very great and accordingly, in order to actually enhance the recording density, each recording pit is formed very small while, on the other hand, the arrangement pitch of the picture elements of the sensor is greater than the arrangement pitch of the recording pits. Therefore, as the imaging optical system, use has heretofore been generally made of an enlarging system such as a microscope objective lens of magnification of the order of 3-20 times.
However, in the microscope objective lens, correction of chromatic aberration is usually effected in the wavelength range of 400-700 nm so that the lens may display a good performance under white illumination such as that of a halogen lamp, and therefore, the number of lenses used is great and the dimensions of the entire lens system are larger than might be desirable.
However, the light source used during the reproduction of the optically recorded information as described above is a substantially monochromatic light source such as a semiconductor laser or a light-emitting diode and therefore, using a microscope objective lens as the imaging optical system is of excessive quality relative to the purpose, and a microscope objective lens of suitable magnification is great in focal length and has a large full optical length. This hinders the compactness and light weight of the reproducing apparatus, and the cost thereof cannot be reduced sufficiently.
To solve the above-noted problems peculiar to the prior art, it is an object of the present invention to provide an imaging single lens which is compact and light in weight while maintaining a good imaging performance in conformity with the purpose of its use and in which a first surface is an axis-symmetrical non-spherical surface having a radius of curvature r.sub.1 in the vicinity of the optical axis, a second surface is a spherical surface having a radius of curvature r.sub.2, the center thickness is d, the non-spherical shape of the first surface is a shape displaced toward the object side from the center to the marginal portion relative to a reference spherical surface of radius r.sub.1, and as p.ident.-r.sub.2 /d, p is 1 or a value approximate thereto.